Since polypropylene nonwoven fabrics are excellent in air permeability, flexibility and lightweight properties, they have been broadly applied to various uses. On that account, for the nonwoven fabrics, various properties have been required according to the uses, and besides, improvement in the properties has been required.
On the other hand, the polypropylene nonwoven fabrics are inherently hydrophobic, so that when they are used for top sheets of sanitary materials such as paper diapers and sanitary napkins, wipes, etc., it is essential to subject the polypropylene nonwoven fabrics to hydrophilic treatment.
As methods for imparting hydrophilicity to the polypropylene nonwoven fabrics, there have been proposed many methods, such as a method of treating a surface of a polypropylene nonwoven fabric with a treating agent containing plural nonionic surface active agents (e.g., patent literature 1: Japanese Patent Laid-Open Publication No. 107131/2007, patent literature 2: Japanese Patent Laid-Open Publication No. 52752/2003), a method of heat treating a spunbonded nonwoven fabric containing a surface active agent for at least 30 seconds (patent literature 3: Japanese Patent Laid-Open Publication No. 211350/1988), and a method of incorporating a surface active agent into sheaths of composite fibers (patent literature 4: Japanese Patent Laid-Open Publication No. 221448/1990)
The method of treating a surface of a polypropylene nonwoven fabric with surface active agents is excellent in initial hydrophilicity, but the surface active agents are apt to bleed out, and when the nonwoven fabric is repeatedly used, its hydrophilicity is liable to be lowered. In the method of incorporating a surface active agent into a polypropylene nonwoven fabric, appearance of hydrophilicity takes time, and there is a fear of poor initial hydrophilicity. In particular, a nonwoven fabric comprising a propylene homopolymer needs a long time before it exhibits hydrophilicity, and besides, when the nonwoven fabric is heat-treated in order to laminate it with another member, recovery of hydrophilicity needs several days. Therefore, in order to use the nonwoven fabric for top sheets of sanitary materials such as paper diapers and sanitary napkins, further improvement in hydrophilicity is desired.